A vehicle wheel is a safety-critical component and must therefore permanently withstand high fluctuating mechanical loads during operation. Conventional steel wheels are composed of a wheel disc, which ensures the connection to the wheel hub, and a wheel-rim band for holding a tyre, which wheel-rim band is connected in encircling fashion at the inside to the wheel disc. Wheel discs are nowadays manufactured on multistage presses with up to 11 stages. Here, use is made almost exclusively of microalloyed and dual-phase steels with a strength of 400 to 600 MPa. Aside from the fatigue strength of steel wheels, weight plays an essential role; this has an effect on material costs, non-suspended masses, rotationally moving masses and fuel consumption.
That which has been stated above self-evidently also applies to numerous other components, in particular in the chassis region of motor vehicles. The present invention therefore relates to the production of any components. Below, therefore, the production of a wheel disc is mentioned merely by way of example.
In order, nowadays, to attain further weight advantages in the case of the wheel disc, material with a relatively high yield strength and adequate vibration resistance must be used in order to ensure an adequate fatigue strength of the wheels, and furthermore, the loss of rigidity in the case of relatively small sheet-metal thickness must be compensated by way of geometrical adaptations, for example relatively thick embossments. Further geometrical adaptations are necessary for example in the region of the bolt holes for receiving the wheel bolts. With increasing strength of the steel materials (carbon steel), however, it is generally the case that the degree of formability decreases; this has already been almost fully exploited in the case of current wheel discs. Therefore, the further lightweight construction potential of cold-formed solutions appears to be limited.
In the vehicle/body construction sector, in addition to cold forming processes, use is also made, inter alia, of hot forming processes. Through the use of hot forming, the demand for high formability with simultaneously high strength of the resulting components can be satisfied. Forming methods which are performed with the involvement of a prior heat treatment of the workpiece, for example in a separate furnace, are well known from the prior art. In particular, hot forming and press hardening are known here.
The hot forming of blanks (plate-shaped workpieces) composed of higher-strength and ultra-high-strength steels for the production of a wide variety of components is performed for example by virtue of the workpiece being brought to a temperature above the austenization temperature, and forming being performed immediately thereafter in a forming tool.
It is furthermore known for hot-formed components to be press-hardened. Press-hardening is based on the principle of the workpiece, which has been previously heated to a temperature above the austenization temperature, being intensely cooled in the forming tool, for example using a cooling device in the form of cooling ducts in the die and in the punch of the forming tool. The rapid cooling causes the microstructure to be converted entirely into martensite, and to thus be fully press-hardened (full press hardening).
Also known from the prior art or composite materials which have multiple steel layers which are fixedly connected to one another. The steel layers of such multi-layer steel components or steel blanks are non-detachably connected to one another by roll-bonding (hot rolling).
DE 10 2012 100 278 A1 has disclosed a two-layer composite material in which the individual steel layers have different strengths. Proceeding from the starting material, that is to say the roll-bonded metal sheet, a rotationally symmetrical component is produced by cold forming.
DE 10 2008 048 389 A1 has disclosed a three-layer composite material, wherein the roll-bonded starting product is likewise formed by cold forming. In this case, the internal steel layer has a lower strength than the two external steel layers.
DE 10 2008 022 709 A1 has disclosed a likewise three-layer composite material, in which the individual layers may have different strengths. In this case, the forming is performed by hot forming.